This invention relates to the manufacture of polishing cloths of the class to be used with an abrasive powder or slurry for polishing various work pieces on a lapping machine.
Japanese Patent Publication No. 30158/1979 discloses a polishing cloth of the above type made of microporous polyurethane sheet materials. The microporous polyurethane sheet is produced using the well-known wet coagulation process by impregnating a substrate fabric with a solution of polyurethane elastomer in a solvent, and immersing the impregnated fabric in a liquid which is a nonsolvent for polyurethane elastomer but miscible with said solvent so that polyurethane solution is coagulated into a microporous structure. The polishing cloth is produced by compressing the resulting sheet at a temperature above the softening point of the polyurethane elastomer to impart the sheet with an adequate hardness.
Experiments have shown, however, that the resulting polishing cloths produced by the above known process have certain disadvantages and thus are far from satisfactory. The compression under heat tends to partially collapse pores and reduce their sizes in terms of greatly decreased air-permeability. In addition, the pores may be easily clogged with abrasive powder particles with the result being decreased polishing efficiency and also increased scratches on the work piece surfaces. If the polishing cloth is too soft, it is difficult to maintain flatness accuracy free from relief on the polished surface particularly when the polishing is carried out at high speeds under high pressures.